Of the known electronic display technologies, organic light emitting devices (OLEDs) are of considerable interest for their potential role in the development of full color, flat-panel display systems. OLEDs are comprised of several organic layers in which at least one of the layers can be made to electroluminesce by applying a voltage across the device (see, e.g., Tang, et al., Appl. Phys. Lett. 1987, 51, 913 and Burroughes, et al., Nature, 1990, 347, 359). When a voltage is applied across a device, holes and electrons migrate toward their respective oppositely charged electrodes. Recombination of the hole and electron is accompanied by radiative emission, thereby producing electroluminescence.
Movement of charge across an OLED is typically facilitated by inclusion of organic carrier transport layers. Organic materials used in this capacity generally are characterized as having high charge mobility and a low barrier to charge injection. Despite these favorable charge transporting characteristics, conductivity remains relatively low, especially in comparison to doped inorganic semiconductor devices. Consequently, OLEDs often have undesirable high operating voltages.
In contrast with doped inorganic semiconductor light emitting diodes or lasers, nominally undoped OLEDs have low intrinsic carrier concentrations. Intentional doping of the organic charge transporting layer has been studied as a possible means for remedying this deficiency and improving conductivity and power efficiency. p-Type doping of organic hole transporting materials with the organic compound tetrafluoro-tetracyano-quinodimethane (F4-TCNQ) has been reported in Blochwitz, et al., Appl. Phys. Lett., 1998, 73, 729; Pfeiffer, et al., Appl. Phys. Lett., 1998, 73, 3202; Zhou, et al., Appl. Phys. Lett., 2001, 78, 410; and Blochwitz, et al., Organic Electronics, 2001, 2, 97. Similarly, n-type doping is the subject of Nollau, et al., J. Appl. Phys., 2000, 87, 4340 which reports doping of naphthalenetetracarboxylic dianhydride (NTCDA) with bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF). OLEDs containing both p- and n-type doped hole and electron transporting layers, respectively, are reported in Huang, et al., Appl. Phys. Lett., 2002, 80, 139. Doped polymeric hole transporting layers are reported in Yamamori, et al., Appl. Phys. Lett. 1998, 72, 2147; Yamamori, et al., J. Appl. Phys., 1999, 86, 4369; and JP 11283750. Electron transporting layers doped with metals are reported in Kido, et al., Appl. Phys. Lett., 1998, 73, 2866; WO 01/41513; EP 1089361; and EP 1011155.
Currently, few compounds have been identified as suitable for enhancing conductivity in OLEDs. Known dopants are typically useful only in combination with a narrow range of organic charge transporting materials. In this regard, new dopants are needed, including more versatile dopants that can be readily adjusted or “tuned” to energetically fit with any given charge transporting material. Identification of new dopants can result in improved OLEDs having higher power efficiency, lower driving voltages, more efficient charge injection, and improved conductivity. The compositions, methods, and devices described herein help fulfill these and other needs.